Fixes: NB#173388 - Contacts are not displayed with avatar when they are synchronized

Bug: NB#173388,BMC#5879
RevBy: Mathias Hasselmann
Details: Translate the QContactThumbnail that QtVersit creates of the
vCard::PHOTO entry to a QContactAvatar by saving contained image to local
cache dir.

RevBy: TrustMe
Details: Initially we limited to nco:Role instead of rdf:Resource to only hit
"owned" properties, but this causes trouble with properties like nco:fullname
which for instance also implements dc:title.

Changes: Prevent name class between SopranoLive::BackEnds::Tracker::tracker() and SopranoLive::Ontologies::tracker.

RevBy: TrustMe
Details: Don't include <QtTracker/Tracker> to prevent
SopranoLive::BackEnds::Tracker::tracker() from being imported into the global
namespace, which will cause name clashes with SopranoLive::Ontologies::tracker.